Spoiled tea?

I'm very confused about a high fired TGY I bought last month or so...it tasted great initially but now it has a very unpleasant sour taste. I kept it stored in a tea canister with tight fitting lid, inside a drawer in my desk...so it was away from bad odours, light, and excessive air exposure. It seems like the tea spoiled somehow anyway. I don't see any obvious signs of decay on the leaves.

Anyone have any idea why the taste would suddenly change like this for the worst?

It's not the water, it tastes good straight out of my kettle and with other tea...

I know that some of my high fire teas improve with aging for a few months, because they can be bitter and scorched tasting at first. I sometimes have been extra careful with a first tasting session when I open the bag, think, how nice!, then soon afterwards am brewing the same tea less mindfully and think, 'not so nice', but a few months later come back to it and I like it again.

I had kept the open vacuum packs of some of my taiwanese oolongs in those japanese tin canisters with paper on the outside. After a while I noticed the canisters seemed to give a sour metallic taste to the teas so removed them and not soon after the teas tasted a lot better again.

So I left this tea alone pretty much all summer and have given it a try a couple of times in the past week...

And it tastes so much better The horrible sour taste is gone and is replaced with a bit of plummy sweetness. I suppose it was going through an awkward phase as the roast faded, but I was really shocked at how bad it was for a while...